Calcium phosphate materials including hydroxyapatite, fluoroapatite, and tricalcium phosphate are widely used as living hard tissue replacements such as artificial bones and dental roots because their sintered bodies are non-toxic and likely to bond with the bone in a living system. Sintered calcium phosphate, however, is not necessarily satisfactory in practice because it is neither mechanically strong nor tough. Sintered calcium phosphate tends to break or chip away when it is shaped to a configuration conforming to a deficient site in a living system, fitted and embedded in the site.
One solution is the use of a core which is coated with calcium phosphate as disclosed in Japanese Patent Publication No. 39533/1983. However, calcium phosphate coatings do not bond firmly to the core. Japanese Patent Application Kokai No. 161971/1988 teaches to enhance the bond between a calcium phosphate coating and a core by interposing ceramics fibers having one end embedded in the core and another end protruding into the coating. Nevertheless, the strength and toughness of calcium phosphate coatings themselves are unsatisfactory.
Various mixtures were proposed in order to increase the strength of calcium phosphate. For example, Japanese Patent Publication No. 40803/1982 discloses an admixture of apatite and at least one member of particulate SiO.sub.2, Al.sub.2 O.sub.3, and the like. Japanese Patent Application Kokai Nos. 96359/1987 and 96360/1987 discloses a mixture of tricalcium phosphate and particulate amorphous quartz. Japanese Patent Application Kokai No. 297254/1987 discloses a mixture of calcium phosphate and particulate ceramic material. These mixtures show increased flexural strength, but are insufficient in toughness.
Japanese Patent Application Kokai No. 57971/1984 discloses fibrous or acicular apatite which is reinforced with mineral fibers. This composition, however, is impractical in that manufacture of apatite in fiber or needle form is very difficult and mineral fibers having affinity thereto must be separately prepared. It is a cumbersome operation to sinter apatite in close contact with mineral fibers under pressure at a relatively low temperature of lower than 800.degree. C. under conditions to prevent substantial escape of moisture. In addition, such low-temperature sintering results in a low relative density. Many problems must be overcome before this composition can be used in practice.
Japanese Patent Application Kokai No. 162676/1987 discloses a composite apatite material of whisker reinforcement type in which mullite whiskers are simultaneously grown during sintering of apatite. However, this reference pays no attention to the grain size of apatite and the amount and dimensions of whiskers. More particularly, according to this reference, a reinforced composite material is prepared by mixing calcium phosphate powder and a mixture of powder silica and powder alumina as a mullite-forming material, and sintering the resulting mixture at 1300.degree. to 1350.degree. C. The present inventors found that when the mixture was sintered at such temperatures, whiskers precipitated only in an amount occupying less than 0.1% by area of a cross section of the sintered body. The sintered body is not tough enough to prevent cracking and chipping upon application to a deficient site in a living system.
Japanese Patent Application Kokai No. 166772/1988 discloses whisker-reinforced ceramics. Whiskers of SiC or Si.sub.3 N.sub.4 are dispersed in ceramics. Examples of the ceramics disclosed therein are Si.sub.3 N.sub.4, SiAlON, zirconia, alumina, and calcium phosphate. Japanese Patent Application Kokai No. 151652/1988 discloses a biomedical implanting material comprising calcium phosphate matrix and 40 to 85% by weight of the material of SiC whiskers having an average length of 2 to 10 .mu.m. Undesirably SiC and Si.sub.3 N.sub.4 whiskers have poor biological affinity.
There is a need for overcoming the above-mentioned problems of the prior art sintered calcium phosphate materials.